Fire performance cable lottery

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We are all aware of the important role that is played by fire resistant cables in providing circuit integrity in critical life safety, fire fighting applications and other emergency circuits. Should any of the cables performing these operations fail, the potential for loss of life and property is substantially increased, so we rely heavily on standards to define and set the minimum requirements for the cables performance and resistance to a fire situation. Dave Winship, sales manager for UK/export at Wrexham Mineral Cables, explains:

The dynamics of a fire are such they all start with increased temperature, then flame which will continue to spread until the available oxygen and fuel is consumed or the fire is tackled and extinguished. During the first stage the increased temperature will release smoke by the process of pyrolysis a thermo-chemical decomposition of the organic materials. This smoke is actually a gas which consists of un-burnt particles of energy, this gas is highly flammable and volatile, more so as the temperature increases only requiring a spark to ignite if oxygen is present. Once flames eventually break out the contents of the building would fuel the fire to attack the structure of the building, in this stage there is a very strong likelihood of explosions causing debris to impact directly on to equipment and importantly the fire resistant cables that are supporting life critical equipment. From this it is clear that in order for a cable to remain operational in a fire it needs to withstand fire, direct mechanical impact and water spray simultaneously.

Confusion

It has been well publicised and documented over the years the confusion surrounding the actual performance and test criteria of cables and the relevant standards. One such issue is the test criteria of BS EN 50200:2006 ‘Method of test for resistance to fire of unprotected small cables for use in emergency circuits’, which is described as exposure to flame (842 Deg C), shock and water spray. This standard applies to cables with an overall diameter not exceeding 20mm. On many occasions in discussions with consultants, engineers and electrical contractors regarding this standard I have heard them use the term ‘impact’  in place of the correct term ‘shock’ when discussing the perceived performance of cables to this standard. Even the Loss Prevention Certification Board (LPCB) have recently amended the listings on their website to correctly define the test criteria as ‘shock’ in place of ‘impact’. The standard defines in great detail the construction of the test equipment and the procedure for mounting the cable for testing. Basically the cable is mounted onto a non combustible board which is then fastened rigidly to two horizontal steel supports, with the top support slightly above the upper edge of the board.  The shock producing device is only allowed to strike the steel support which is fastened to a rigid frame by four bonded rubber bushes, surely these will act as shock absorbers thus greatly reducing the amount of mechanical shock transferred to the cable? At no point during the test is there any direct impact on the cable.

Requirements
Considering the importance for cables to withstand a direct mechanical impact while subjected to fire, the test procedure defined by BS EN 50200:2006 would seem to be farcical! The requirement for this has been clearly defined within the standard BS8491:2008 which calls for a direct mechanical impact to the cable while subjected to fire & water, though this standard only applies to cables with an overall diameter greater than 20mm, cables conforming to both standards are in the same fire at the same time protecting the same people so why would the fire not cause impacts directly on all cables regardless of diameter?  Clearly there would be substantial risk of direct impact to all cables as the fire develops causing debris to crash down and objects to explode.

BS 8491:2008 has a clear bias towards the use of polymeric cables by limiting the overall cable diameter to over 20mm, this has no relevance to the fire performance or current carrying capacity of the cable.  This also encourages the loading of unnecessary polymers into the cable design so that cables fall into the correct size category to be tested to the standard, theses polymers will add additional fuel to the fire while producing larger volumes of flammable smoke when exposed to radiated heat.

BS EN 61034-2:2005 amendment A1:20013 Measurement of smoke density of cables burning under defined conditions, commonly know as the 3m cube test, calls for one metre long cables samples to be burned. It’s not until you study diagram detailing the test equipment that you can determine the burner has a length of only 240mm. If the standard is testing 1m cable samples then in my opinion the burner should be 1m long. The test measures the percentage of light transmitted and received through the smoke, in order to pass the minimum light received (transmittance) should be no less than 60%, this is a huge volume of smoke when you consider the result for four samples of mineral insulated cable with a diameter of 9.1mm over the LSZH sheath achieves 98.6% light received.

Mineral insulated cables
In summary, if you require a cable to survive and safely carry current to critical circuits during a emergency fire situation without adding substantial load to the fire or producing high concentrations of smoke then specify mineral insulated cables which have all of the properties required to survive a real fire rather than a controlled laboratory fire test.

 

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